There is no advantage over the Greenmarket other than price (hard to calculate exactly but it is generally more affordable for the amount). The produce is local, if you care to support smaller producers, and picked that day or the day before, which are things you can't get consistently at any grocery store. Some CSAs (the supplier for Park Slope and Clinton Hill CSAs for example) offer quite exotic selections, i.e. heirloom varieties and not-your-usual herbs, so there is that advantage as well.

in some csas you have a choice on some things (either 3 eggplants or a bag of green beans, say) but generally the risk is that some veggies/fruits won't be what you want. You can give them to friends or leave them at the pick-up. It's kind of part of the gamble-- something that some people like and others don't.

i think the huge advantage over any type of supermarket (even in this case, the Coop) that the CSA offers is the idea of removing yourself from the marketing/consumer train. you ar egoing directly to the farmer. no middle man, in other words. more direct. simpler, in the plainest terms. i do shop at the Coop but i appreciate CSAs.